1. Field of the Invention
The present invention relates to a pyrazine compound which is useful as a charge transporting material for an electrophotographic photoconductor, and also relates to an electrophotographic photoconductor comprising the pyrazine compound as a charge transporting material.
2. Discussion of Background
Conventionally inorganic photoconductive materials such as selenium, selenium-tellurium alloys and zinc oxide have been used as an effective component for a photoconductive layer of an electrophotographic photoconductor. Recently, however, electrophotographic photoconductors employing organic photoconductive materials have been studied and developed, and some of them are now used in practice. Most of such organic photoconductors now used in practice are of a so-called function-separated layered type comprising a charge generation layer and a charge transport layer which are overlaid. These organic photoconductors are much more improved with respect to the photosensitivity and the life thereof in comparison with conventional inorganic photoconductors, and therefore the development activities concerning such organic photoconductors are actively made from the viewpoints of the advantages thereof that they are inexpensive, safe for use in practice and have varieties of properties.
Such a layered electrophotographic photoconductor generally comprises an electroconductive support, a charge generation layer formed thereon comprising a charge generating material such as a pigment or dye, an a charge transport layer formed on the charge generation layer, comprising a charge transporting material such as hydrazone or pyrazoline. Such a charge transporting material has electron donating properties, so that a layered electrophotographic photoconductor using such a charge transporting material is of a positive-hole moving type and therefore exhibits photosensitivity when negatively charged. However, corona discharging used in such negative charging is much more unstable than that used in positive charging. Furthermore, such negative charging tends to produce ozone and nitrogen oxides in an amount of about 10 times that at the time of positive charging, and also tends to cause chemical and physical deterioration in the surface of the photoconductor because of the adsorption of the produced ozone and nitrogen oxides on the surface thereof, thus causing environmental pollution problems as well.
Furthermore, a positive charging toner is required for the development of latent electrostatic images formed on the negative charging photoconductor. However, the production of a positive charging toner is difficult in view of the triboelectrical charging series thereof with respect to ferromagnetic carrier particles to be used in combination with the toner.
In addition, in a two-component, high resistivity magnetic brush development method, a developer comprising a negative charging toner is much more stable and has more freedom in the formulation design thereof than the developer comprising a positive charging toner. Also from this point of view, a positive charging photoconductor is more practical and has a wider application scope than the negative charging photoconductor.
Under such circumstances, the use of a photoconductor comprising an organic photoconductive material under positive charging has been proposed.
When an organic electrophotographic photoconductor is fabricated by overlaying a charge transport layer on a charge generation layer, for example, 2,4,7-trinitro-9-fluorenone is employed as a charge transporting material for the charge transport layer since it has a great charge transporting performance. However, this material is carcinogenic and therefore extremely unsuitable for use in practice in view of industrial hygiene.
U.S. Pat. No. 3,615,414 discloses a positive charging photoconductor which comprises a thiapyrylium salt serving as a charge generating material and polycarbonate serving as a binder agent which constitute a eutectic crystal complex. This photoconductor, however, has the shortcomings that considerable memory development takes place and ghost images are formed quite easily.
In order to eliminate the above-mentioned shortcomings, a layered photoconductor for positive charging may be proposed, which comprises an upper surface layer serving as a charge generation layer comprising a charge generating material which is capable of generating positive holes and electrons, and a lower layer serving as a charge transport layer comprising a charge transporting material which has a positive-hole-transporting performance. This layered photoconductor, however, has the shortcomings that the electrophotographic performance deteriorates in the course of the preservation of the photoconductor and also in the course of image formation, thus producing images with low image density. This is because the layer comprising the charge generating material is situated on the top of the photoconductor and therefore the charge generating material which is weak and susceptible to external actions, for example, coherent light beam radiation, such as ultraviolet light radiation, corona discharging, humidity, and mechanical frictions, is easily affected by such external actions.
In the conventional photoconductor for negative charging provided with the charge transport layer as the top layer thereof, the photoconductor is rarely affected by the above-mentioned external actions, and the charge transport layer rather serves to protect the inner charge generation layer.
For the protection of the charge generation layer, a protective layer made of an insulating and transparent resin may be proposed. However, such a protective layer may block the electrons generated when the charge generation layer is radiated, so that the radiation effect is redued, and when the protective layer is thick, the photosensitivity of the photoconductor is significantly decreased.
Under such circumstances, various trials have been made and now are being made to obtain a photoconductor for use with positive charging. However, photoconductors proposed so far have various problems to be solved, in particular, with respect to photosensitivity, memory development phenomenon and industrial hygiene.